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Multi-wavelength campaign on NCG 7469R. Middei, S. Bianchi, M. Cappi, Pierre-Olivier Petrucci, F. Ursini, N. Arav,

E. Behar, G. Branduardi-Raymont, E. Costantini, B. de Marco, et al.

To cite this version:R. Middei, S. Bianchi, M. Cappi, Pierre-Olivier Petrucci, F. Ursini, et al.. Multi-wavelength campaignon NCG 7469. Astronomy and Astrophysics - A&A, EDP Sciences, 2018, 615, pp.A163. �10.1051/0004-6361/201832726�. �hal-03014197�

Astronomy&Astrophysics

A&A 615, A163 (2018)https://doi.org/10.1051/0004-6361/201832726© ESO 2018

Multi-wavelength campaign on NCG 7469

IV. The broad-band X-ray spectrum

R. Middei1, S. Bianchi1, M. Cappi2, P.-O. Petrucci3, F. Ursini2, N. Arav4,5, E. Behar4,6, G. Branduardi-Raymont7,E. Costantini8, B. De Marco9, L. Di Gesu10, J. Ebrero11, J. Kaastra8,12, S. Kaspi13, G. A. Kriss14, J. Mao8,

M. Mehdipour8, S. Paltani10, U. Peretz4, and G. Ponti15

1 Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italye-mail: riccardo.middei@uniroma3.it

2 INAF - IASF Bologna, via Gobetti 101, 40129 Bologna, Italy3 Univ. Grenoble-Alpes, CNRS, IPAG, 38000 Grenoble, France4 Department of Physics, Technion, Haifa 32000, Israel5 Department of Physics, Virginia Tech, Blacksburg, VA 24061, USA6 Department of Astronomy, University of Maryland, College Park, USA7 Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK8 SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands9 Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warsaw, Poland

10 Department of Astronomy, University of Geneva, 16 Chemin d’Ecogia, 1290 Versoix, Switzerland11 European Space Astronomy Centre, Villanueva de la Cañada, PO Box 78, 28691 Madrid, Spain12 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands13 School of Physics & Astronomy and Wise Observatory, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel-Aviv

University, Tel-Aviv 69978, Israel14 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA15 Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany

Received 29 January 2018 / Accepted 19 March 2018

ABSTRACT

We conducted a multi-wavelength 6-month campaign to observe the Seyfert Galaxy NGC 7469, using the space-based observatoriesHST, Swift, XMM-Newton and NuSTAR. We report the results of the spectral analysis of the seven simultaneous XMM-Newton andNuSTAR observations. The source shows significant flux variability within each observation, but the average flux is less variableamong the different pointings of our campaign. Our spectral analysis reveals a prominent narrow neutral Fe Kα emission line in allthe spectra and weaker contributions from Fe Kβ, neutral Ni Kα, and ionized iron. We find no evidence for variability or relativisticeffects acting on the emission lines, which indicates that they originate from distant material. In the joint analysis of XMM-Newtonand NuSTAR data, a constant photon index is found (Γ = 1.78 ± 0.02) together with a high energy cut-off Ecut = 170+60

−40 keV. Adoptinga self-consistent Comptonization model, these values correspond to an average coronal electron temperature of kT = 45+15

−12 keV and,assuming a spherical geometry, an optical depth τ = 2.6 ± 0.9. The reflection component is consistent with being constant and thereflection fraction is in the range R = 0.3−0.6. A prominent soft excess dominates the spectra below 4 keV. This is best fit with asecond Comptonization component, arising from a warm corona with an average kT = 0.67 ± 0.03 keV and a corresponding opticaldepth τ = 9.2 ± 0.2.

Key words. galaxies: active – galaxies: Seyfert – quasars: general – X-rays: galaxies

1. Introduction

Active galactic nuclei (AGN) are among the brightest sourcesin the Universe and they account for a large percentage of theX-ray photons we observe in the sky. It is commonly acceptedthat AGN are powered by matter accreting onto a supermas-sive black hole (SMBH). In the innermost region of the hostgalaxy, a SMBH is surrounded by a disc of spiralling matterthat is responsible for its optical-UV emission, while the phys-ical origin of the higher energetic photons still remains elusive.According to the commonly accepted scenario, X-rays are pro-duced in a region close to the central black hole (BH), theso-called hot corona (e.g. Haardt & Maraschi 1991, 1993; Haardtet al. 1994), in which seed optical-UV photons arising from the

accreting disc interact with hot thermal electrons through aninverse Compton process. This process reproduces the power lawshape we commonly observe in AGN spectra (e.g. Guainazziet al. 1999; Bianchi et al. 2009; Marinucci et al. 2014). More-over, a high energy cut-off is observed in different AGN spectra(e.g. Perola et al. 2002; De Rosa et al. 2002; Guainazzi et al.2010; Brenneman et al. 2014; Marinucci et al. 2014; Lohfinket al. 2015; Ursini et al. 2016; Tortosa et al. 2017, 2018a; Porquetet al. 2018), and this is another signature of the thermal Comp-tonization acting in the hot corona (Haardt & Maraschi 1991,1993). Furthermore, the primary continuum emission can bemodified by a Compton reflection from the disc, from far-ther material or by absorption from neutral or from ionizedgas.

Article published by EDP Sciences A163, page 1 of 10

A&A 615, A163 (2018)

As an additional hallmark of AGN activity there is contin-uum variability. Flux variations are indeed observed on severaltimescales, from hours and days (e.g. Ponti et al. 2012) up toyears and decades (e.g. Vagnetti et al. 2011, 2016; Middei et al.2017). Rapid variability not only is of primary importance toinvestigate the X-ray emission, but it can be also used to estimatethe SMBH mass (e.g. McHardy et al. 2006; Ponti et al. 2012) andas a luminosity distance estimator (La Franca et al. 2014).

Long multi-wavelength monitorings of single nearby AGNproduced outstanding results (e.g. Kaastra et al. 2011, and therelated series of papers on Mrk 509). The target of our obser-vational campaign is NGC 7469, which is a luminous Seyfertgalaxy (Lbol ∼ 1045 erg s−1; Behar et al. 2017) at z = 0.016268(Springob et al. 2005). Using reverberation mapping, Petersonet al. (2014) found that NGC 7469 hosts a BH with a mass of1.1 ± 0.1 × 107 M� and an Eddington ratio of the order of 0.3.In X-ray wavelengths, this Seyfert Galaxy was first observed bythe Uhuru satellite (Forman et al. 1978) in the seventies, andit was subsequently studied by many other observatories thatfound this source to have a complex X-ray emission. Since theEXOSAT observation, we know that its X-ray spectrum displaysan excess in the soft band (Barr 1986). Other authors (Turneret al. 1991; Brandt et al. 1993; Guainazzi et al. 1994; Nandra et al.1998, 2000; De Rosa et al. 2002) analysed this source using dataobtained by Einstein, ROSAT, ASCA, RXTE and BeppoSAX.

NGC 7469 was also studied more recently: Petrucci et al.(2004) investigated the UV/X-ray variability, Scott et al.(2005) analysed its simultaneous X-ray, far-ultraviolet, and near-ultraviolet spectra using Chandra, FUSE and STIS, while Patricket al. (2011) studied this source taking advantage of Suzakuobservations. Previous XMM-Newton data were analysed inBlustin et al. (2003) and De Marco et al. (2009), while someresults from the 2015 observational campaign have been pre-sented in Behar et al. (2017) and Peretz et al. (2018).

This paper focuses on the seven simultaneous XMM-Newtonand NuSTAR observations of our campaign, and it is organizedas follows. Section 2 presents the NGC 7469 temporal analysis.Section 3 focuses on the data reduction. Sections 4 and 5 reporton the spectral analysis. Section 6 contains the discussion of theresults, and in Sect. 7 a summary of this work is reported.

2. Observations and data reduction

The spectral analysis presented in this work is based on XMM-Newton (Jansen et al. 2001) and NuSTAR (Harrison et al. 2013)observations of NGC 7469 belonging to the multi-wavelengthcampaign first described by Behar et al. (2017). The two satel-lites observed the source simultaneously between June 12 andDecember 28, 2015. The seven observations are spaced by dif-ferent time intervals, allowing us to study flux and spectralvariations on different timescales, see Table 1.

XMM-Newton data were obtained using the EPIC cameras(Strüder et al. 2001; Turner et al. 2001) in the Small Windowoperating mode and they were processed taking advantage of theXMM-Newton Science Analysis System1 (SAS, Version 15.0.0).Because of its larger effective area with respect to the two MOScameras, we only report the results for the PN instrument. Weextract spectra from circular regions of 50 arcsec radius for thebackground, and 40 arcsec radius for the source. These regionsare selected by an iterative process that maximizes the signal-to-noise ratio (S/N; Piconcelli et al. 2004). All the spectra were

1 ‘Users Guide to the XMM-Newton Science Analysis System’, Issue13.0, 2017 (ESA: XMM-Newton SOC).

Table 1. Observation ID, start date, and net exposure time (ks) for eachsatellite, and for the seven observations analysed in this work.

Obs. satellites Obs. ID Start date Net exp. (ks)

XMM-Newton 0760350201 2015-06-12 63NuSTAR 60101001002 2015-06-12 21

XMM-Newton 0760350301 2015-11-24 59NuSTAR 60101001004 2015-11-24 20

XMM-Newton 0760350401 2015-12-15 59NuSTAR 60101001006 2015-12-15 22

XMM-Newton 0760350501 2015-12-23 62NuSTAR 60101001008 2015-12-22 23

XMM-Newton 0760350601 2015-12-24 65NuSTAR 60101001010 2015-12-25 21

XMM-Newton 0760350701 2015-12-26 67NuSTAR 60101001012 2015-12-27 21

XMM-Newton 0760350801 2015-12-28 70NuSTAR 60101001014 2015-12-28 23

rebinned in order to have at least 30 counts for each bin and not tooversample the spectral resolution by a factor greater than three.

NuSTAR data were reduced taking advantage of the standardpipeline (nupipeline) in the NuSTAR Data Analysis Software(nustardas release: nustardas_14Apr16_v1.6.0, part of the hea-soft distribution2), adopting the latest calibration database. TheNuSTAR observatory carries in its focal plane two modules, Aand B, corresponding to the hard X-ray detectors FPMA andFPMB. Spectra and light curves were extracted for both modulesusing the standard tool nuproducts. A circular region with radiusof ∼70 arcsec is used to extract the source counts while the back-ground is obtained from a blank area with the same radius, closeto the source. Similar to the XMM-Newton spectra, we binnedNuSTAR spectra to have an S/N greater than five in each spectralchannel, and to avoid oversampling the instrumental resolutionby a factor greater than 2.5. The spectra of the two modules arein good agreement with each other, the cross-normalization con-stant for FPMB with respect to FPMA in all the fits being unitywithin 1%. Spectra were analysed with XSPEC 12.9 (Arnaud1996). All errors reported in the plots account for 1σ uncertainty,while errors in text and tables are quoted at 90% confidencelevel, unless otherwise stated.

3. Temporal analysis

We started investigating the NGC 7469 temporal properties com-puting the light curves for all the observations. We used thestandard command epiclccorr for the XMM-Newton data tocompute corrected for the background light curves in the 0.5–2 keV and 2–10 keV bands, while, we used the nuproductspipeline to compute the NuSTAR light curves in the 10–80 keVband. The XMM-Newton and NuSTAR light curves of all theobservations of our campaign are shown in Fig. 1. The sourceshows a remarkable intra-observation flux variability (e.g. upto ∼60% in the sixth observation for the XMM-Newton in the0.5–2 and 2–10 keV bands) and has significant flux variationson timescales of a few ks. On longer timescales, flux variabilityappears to be less significant. The mean counts for each of the2 NuSTARDAS software guide, Perri et al. (2013),https://heasarc.gsfc.nasa.gov/docs/nustar/analysis/nustar_swguide.pdf

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R. Middei et al.: Multi-wavelength campaign on NCG 7469. IV.

Fig. 1. Light curves corresponding to the seven simultaneous observations performed by XMM-Newton and NuSTAR for NGC 7469, in threedifferent energy bands: 0.5–2 keV (first row), 2.0–10 keV (second row), and 10–80 keV (third row). The fourth row shows the ratio between the soft0.5–2 keV and hard 2.0–10 keV light curves. The axis units are sec and counts for the x-axis and y-axis, respectively. The solid line overlapping allthe light curves in each sub-panel represents the average of the counts for the specific light curve.

seven observations is observed to be, on average, weakly variable(∼15%, ∼9%, and few per cent in the 0.5–2, 2–10, and 10–80 keVbands, respectively).

A convenient analysis tool for variability characterizationis the so-called normalized excess variance (NXS). The NXSis defined as σ2

rms = 1Nµ2

∑Ni=1(Xi − µ)2 − σ2

i , where µ is theunweighted count rate mean within the segment of the lightcurve, N is the number of the good time bins in that segment,and Xi represents the count rate with σ2

i as associated uncer-tainty. We computed the σ2

rms in the 2–10 keV band for allthe observations of our campaign in the 20 ks time bin (seePonti et al. 2012 for more details), finding an average valueσ2

rms = 0.0021 ± 0.0005. A tight correlation between the X-rayvariability of the source and its BH mass has been found byseveral authors (e.g. Nandra et al. 1997; Vaughan et al. 2003;McHardy et al. 2006; Ponti et al. 2012). In particular, adoptingthe relation among σ2

rms and MBH in Ponti et al. (2012), we areable to estimate the BH mass MBH = 1.1 ± 0.1 × 107 M� forNGC 7469, in very good agreement with the reverberation map-ping estimate by Peterson et al. (2014). The soft X-ray appearsto be the most variable band, but hardness ratios do not dis-play a large variation (however significant from a statistical pointof view, see Sect. 6) both within and among the observations(∼8%), see Fig. 1.

Therefore, we decided to use the average spectra of eachobservation in the following spectral analysis.

4. Spectral analysis: data above 4 keV

4.1. XMM-Newton: iron line energy band

We start our spectral analysis adopting a simple power lawmodel for all our XMM-Newton data. This crude fit leaves strongresiduals in the soft X-rays. This soft excess extends up to4 keV and is shown in the top left panel of Fig. 2; this valueis obtained by fitting the data above 4 keV and then plotting

the best-fit model with the whole spectrum, i.e. extending themodel into the soft X-ray band. Therefore, as a first step forour analysis, we decided to characterize the limited energy band4–10 keV. The residuals in this energy band are dominated bystrong emission features, readily identified with the Kα linesfrom neutral and H-like iron, and the possible presence of weakercontributions from neutral Nickel, Fe Kβ, and ionized iron(see Fig. 3).

We therefore adopted the following model to fit the 4–10 keVspectra: zashift× (pexrav+zgauss+zgauss). The pexrav code(Magdziarz & Zdziarski 1995) is adopted to model the primaryemission and any reflected component likely associated with thefluorescent emission line from neutral iron model by the firstGaussian line, while the second Gaussian line accounts for theFe XXVI Lyα at 6.966 keV. The zashift component is used tocorrect the well-known XMM-Newton calibration issue affectingthe EPIC pn (a detailed discussion on this topic can be found inCappi et al. 2016). Within this paper, a zashift correction of about−8 × 10−3 (corresponding to ∼50 eV at 6.4 keV), will always beapplied for all the XMM-Newton spectra. To model the data, welet free to vary among the observations the normalizations ofboth the Fe Kα and the Fe XXVI Lyα lines, the reflection param-eter and the photon index in pexrav, and its normalization. Theiron abundance is also free to vary but tied among the differentobservations. This simple model leads us to a very good best fitin the 4–10 keV band (χ2 = 586 for 567 d.o. f .); see top right sidepanel of Fig. 2.

The Fe Kα and Fe XXVI Lyα lines are both statistically signif-icant (>99.9 per cent according to the F-test), and have a constantflux (3σ level) during the seven observations of our campaignwith average values of 3×10−5 and 4×10−6 ph cm−2 s−1, respec-tively, corresponding to equivalent widths of ∼90 and ∼20 eV(see Fig. 4 for the Fe Kα line); however, De Marco et al. (2009)analysing previous XMM-Newton data found possible hints ofvariability in the Fe XXVI Lyα component. We found both theFe Kα and Fe XXVI Lyα lines are narrow and their intrinsic line

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Fig. 2. Top left panel: seven XMM-Newton spectra shown in red; the best-fit model to the data above 4 keV shown in black. Bottom panel: theresiduals of the seven spectra that are grouped using in Xspec the command ‘setplot group’. A secondary component clearly extends up to 4 keV. Topright panel: best fit to the seven XMM-Newton data and corresponding residuals shown with the basic model (i.e. zashift×(pexrav+zgauss+zgauss)),to which we added other Gaussian lines when weaker emission lines were found in the observation. Bottom left panel: best fit for all NuSTAR spectraabove 4 keV is shown; top subpanel shows the best fit obtained adopting the model that employs xillver (see Sect. 4.2), while the bottom subpanelrefers to the corresponding residuals. In the last panel, the best fit (χ2 = 3041 for 2765 d.o.f.) for the NGC 7469 XMM-Newton and NuSTAR spectra;see Sect. 5 for the model details.

Fig. 3. Residuals (in the observer frame) to a simple power law fit tothe XMM-Newton spectra in the energy range 5–8 keV. Emission linesare clearly present. The residuals of the seven spectra are grouped forplotting purposes.

width is consistent with zero in all the observations. In particular,the line profiles do not show any evidence for the relativistic

effects (see Fig. 3) expected to occur in the innermost regionsof the accretion disc.

The residuals in Fig. 3 suggest the presence of the Fe XXVHeα, Fe Kβ, and the Ni Kα emission lines expected at 6.64–6.7,7.06 and 7.47 keV, respectively, so we tried to fit these linesadding three further Gaussian lines to our best-fit model. How-ever, the inclusion of these lines does not improve the χ2 of the fitsignificantly. In fact, these three lines are very weak: the Fe Kβflux is only an upper limit in all the spectra, the nickel line isa detection only in two observations with an average flux of∼4.6 × 10−6 ph cm−2 s−1, and Fe XXV Heα is detected just inthree observations with an average flux ∼4.0 × 10−6 ph cm−2.Indeed, none of these lines is more significant than 98 per cent,according to the F-test. The best-fit normalizations, or theirupper limits, are reported in Table 2 for all the emission linesdiscussed in this section.

4.2. NuSTAR: the 4–80 keV spectra

Our preliminary analysis on the 4–10 keV XMM-Newton datashows a non-variable and narrow neutral iron line, likelyproduced by Compton-thick gas far from the central SMBH.For the NuSTAR data we adopted the self-consistent modelxillver (García & Kallman 2010; García et al. 2013) to fit boththe neutral iron line and the associated reflection continuum;

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Fig. 4. Fe Kα line flux vs. 2–10 continuum flux for the seven XMM-Newton observations. The average line flux is shown together with thestandard deviation (dashed lines).

Table 2. Best-fit normalizations for the emission lines observed in theXMM-Newton observations.

Obs Fe Kα Fe XXVI Lyα Fe XXV Heα Fe Kβ Ni Kα

1 28.7+1.8−1.8 4.3+1.6

−1.7 <3.12 <5.2 <3.3

2 24.7+2.0−2.0 3.5+2.0

−2.0 <1.7 <6.3 4.3+2.8−3.0

3 27.4+2.0−1.9 6.1+2.0

−1.7 <3.1 <6.9 <4.21

4 27.1+1.9−1.9 2.6+1.9

−1.9 4.2+2.8−2.5 <4.4 4.9+3.0

−3.0

5 23.3+1.9−1.9 2.4+2.0

−2.0 4.1+3.0−2.7 <6.0 <4.9

6 27.2+2.0−1.9 4.4+2.0

−2.0 <4.2 <6.9 <4.0

7 29.4+2.0−1.9 4.7+2.0

−2.0 3.9+2.8−2.8 <5.5 <6.4

Notes. The normalizations are in units of 10−6 ph cm−2.

we use xillver with its ionization parameter fixed to zero. AGaussian emission line is included to model the observed emis-sion line at 6.966 keV and a second Gaussian line is used forthe Fe XXV Heα lines. Strong soft excess features also appearin NuSTAR spectra; thus, similar to what we already performedfor XMM-Newton, we preliminary cut NuSTAR spectra below4 keV. The reflection fraction, the high energy cut-off, normal-izations, and the inter-calibration constant are free to vary, whilethe iron abundance is free to vary but tied among the observa-tions. Following this approach, we obtained the best fit to thedata (χ2 = 1762 for 1665 d.o.f.), and this best fit is plotted in thebottom left panel of Fig. 2. The values of the best-fit parametersare reported in Table 3.

This best-fit model requires a super-solar iron abundanceAFe = 2.8 ± 0.6. The photon index is consistent with being con-stant between the observations and has average value of 1.78 ±0.02 (see Fig. 5, top left panel, showing contour plots). A highenergy cut-off is well constrained only in two observations,with values around 150 keV, and there are some indications of

Table 3. Values from the best-fit model for all the NuSTARspectra analysed in the 4–78 keV band using the best-fit modelxillver+zgauss+zgauss (χ2 = 1762 for 1665 d.o.f.).

Obs Γ Ecut R Normxi A†Fe Flux10−78[keV] (10−4)

1 1.82+0.06−0.06 110+85

−35 0.50+0.14−0.10 1.56+0.10

−0.08 2.8 ± 0.6 7.4+0.4−0.3

2 1.77+0.06−0.06 190+650

−90 0.32+0.11−0.10 1.84+0.07

−0.30 2.8 ± 0.6 8.2+0.7−0.4

3 1.73+0.07−0.07 85+60

−20 0.62+0.16−0.13 1.23+0.09

−0.07 2.8 ± 0.6 6.6+0.4−0.3

4 1.83+0.03−0.05 > 230 0.33+0.08

−0.09 2.14+0.02−0.29 2.8 ± 0.6 7.6+0.6

−0.6

5 1.78+0.05−0.06 > 120 0.34+0.10

−0.10 1.89+0.30−0.20 2.8 ± 0.6 8.1+0.6

−0.3

6 1.75+0.06−0.06 > 110 0.37+0.11

−0.10 1.75+0.30−0.19 2.8 ± 0.6 7.8+0.8

−0.6

7 1.78+0.05−0.05 195+420

−80 0.35+0.10−0.09 1.98+0.30

−0.16 2.8 ± 0.6 8.9+0.6−0.4

Notes. The 10–78 keV flux is in units of 10−11 erg cm−2 s−1, and the ironabundance was let free to vary but tied among the observations.

variability up to higher values in other observations (see samepanel in Fig. 5). However, fitting the NuSTAR spectra tying thehigh energy cut-off among the seven observations yields a valueof 170+60

−40 keV with a χ2 = 1775 for 1671 d.o.f. very similar tothe previous one in which the high energy cut-off was untied.Some hints of variability are also found for the reflection frac-tion in the range 0.3–0.6 (see Fig. 5, top right-hand panel,showing contour plots). The flux of the reflection componentis consistent with being constant when the primary contin-uum varies, which is in agreement with an origin from distantmatter; we obtained larger reflection components for lowerflux states.

Although the iron Kα line does not present any broadeningof its profile, we tested for the presence of a relativistic reflectioncomponent. Therefore, we added a further reflection componentto the best-fit model, accounting for the relativistic effects aris-ing in matter in the innermost regions of the accretion disc. Weused relxill (e.g. García et al. 2014; Dauser et al. 2016). The pho-ton index and the high energy cut-off are tied between relxilland xillver, while the relxill ionization parameter ξ and its nor-malization are free to vary in all the observations. No significantimprovement in terms of χ2/d.o.f. is found: ∆χ2 = 30 for 18 d.o.f.less, corresponding to 80% confidence level according to F-test.No relativistic reflection is required by the NuSTAR data: indeed,in all but three of the observations, the normalization of relxill isconsistent with zero.

According to the standard scenario, hard X-rays are pro-duced by Comptonization, thus as a further investigation, wedecided to model the NuSTAR spectra using a self-consistentComptonization model. In our model we substitute xillver withxillvercp (García et al. 2014; Dauser et al. 2016). This dif-ferent code accounts for the primary emission produced bynthcomp (Zdziarski et al. 1996) and a non-relativistic reflec-tion. The parameters of this new model are treated as in theprevious fit and the hot corona electron temperature is freeto vary. The best fit obtained (χ2 = 1768 for 1658 d.o.f.)adopting xillvercp shows larger values for the photon index withrespect to the previous best-fit model with xillver (on aver-age 0.08). On the other hand, the parameters of the reflection

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20

0 1.6 1.65 1.7 1.8 1.91.75 1.85100 200Ecut (keV)

300 400 500

1.8

1.85

1.9

1.7

0.2

0.4

0.6

0.8

1.8

1.9

40 60 80kThc (keV) kTwc (keV)

100 120 140 0.6

2.6

Gam

ma

Gam

ma

Gam

ma

Ref

lect

ion

Fra

ctio

n

Gamma

2.65

2.7

2.75

0.70.65 0.75

Fig. 5. Contours for the high energy cut-off and photon index for all the NuSTAR observations in the top left panel. These contours account for thedata analysed with xillver and above 4 keV. The best-fit parameters are shown in Table 3. Top right plot: best-fit values for the reflection fractionas a function of the photon index for the various observations. The contours are computed on the NuSTAR data above 4 keV adopting the modelwith xillver. Bottom left panel: contour plots for the photon index and hot electron temperature computed on the NuSTAR data above 4 keV withxillvercp. Last panel: contour plots for the warm corona parameters for all the observations of our campaign.

component are in agreement with the values previously quoted.The best-fit values for the parameters of this fit are shown inTable 4.

In the bottom left panel of Fig. 5, the contour plots for thephoton index and hot electron temperature are reported. Weakvariations in the photon index are observed, while the electrontemperature has a more constant behaviour. To test the variabil-ity of the hot electron temperature, similar to what we alreadyperformed for the high energy cut-off, we fit the NuSTAR spectratying the electron temperature among the observations of thiscampaign. This yielded a measure of kT = 45+15

−12 keV corre-sponding to a best fit (χ2 = 1773 for 1664 d.o.f. ) very similarto the previous value.

5. Spectral analysis: 0.5–80 keV band

As shown in Sect. 4.1 (see also Fig. 2, top left hand panel)the data below 4 keV are characterized by a strong soft excess.In order to properly model the continuum emission associatedwith this excess, we first characterize any discrete emitting and

absorbing feature expected in the 0.5–4 keV band. On one side,these are features that can be directly attributed to the detectorsystematic calibration uncertainties, i.e. issues on its quantumefficiency at the Si K-edge (1.84 keV), and on the mirrors effec-tive area at the Au M-edge (∼2.4 keV). To avoid these issues,we ignored the spectral bins in the energy range 1.7–2.6 keV(see e.g. Kaastra et al. 2011; Di Gesu et al. 2015; Ursini et al.2015; Cappi et al. 2016). We then included all the emission andabsorption features (e.g. due to the warm absorbers) derivedfrom the analysis of the XMM-Newton RGS spectra of our cam-paign by Behar et al. (2017). Since none of these componentsdisplayed significant variability during our campaign (Beharet al. 2017; Peretz et al. 2018), we kept all their parameters fixedin our following fits to the values found from the RGS data(see Behar et al. 2017, for a detailed description of all thecomponents). Some line-like features still remained in thepn spectra, and even if very weak, they result significant in termsof χ2, as a consequence of the high number of counts in the softband. Two narrow Gaussian lines untied and free to vary amongthe observations at ∼0.75 keV and ∼1 keV are enough to correct

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Table 4. Values and errors for the best-fit parameters of all the NuSTARspectra analysed in the 4–78 keV band adopting xillvercp.

Obs. Γhard kThc R τhc Normxicp A†Fe[keV] (10−4)

1 1.89+0.02−0.02 20+15

−4 0.45+0.10−0.10 3.0+0.1

−0.1 1.39+0.18−0.11 2.4 ± 0.4

2 1.83+0.02−0.01 >21 0.27+0.10

−0.09 >3.1 1.75+0.11−0.30 2.4 ± 0.4

3 1.85+0.02−0.02 22+16

−5 0.52+0.11−0.11 2.9+0.1

−0.1 1.25+0.11−0.09 2.4 ± 0.4

4 1.85+0.01−0.02 >20 0.30+0.10

−0.11 >3.3 2.17+0.05−0.30 2.4 ± 0.4

5 1.82+0.01−0.02 >20 0.30+0.09

−0.09 >3.2 1.95+0.20−0.19 2.4 ± 0.4

6 1.81+0.01−0.02 >26 0.29+0.10

−0.09 >2.8 1.98+0.30−0.18 2.4 ± 0.4

7 1.83+0.01−0.01 37+140

−15 0.30+0.10−0.10 2.2+4.0

−0.1 1.94+0.20−0.17 2.4 ± 0.4

Notes. Optical depth estimates are obtained following Beloborodov(1999), see Sect. 5 for details. The iron abundance is free to vary buttied among the observations.

these residual narrow features (see also e.g. Kaastra et al. 2011;Di Gesu et al. 2015; Ursini et al. 2015; Cappi et al. 2016).

At first, we model the soft X-ray emission with a phe-nomenological continuum model, such as a power law or ablack body. However, in both cases we do not get an accept-able fit, obtaining χ2 = 3542 for 2793 d.o.f. and χ2 = 7430 for2793 d.o.f., respectively. We then tried to reproduce the softexcess via two self-consistent models: blurred relativistic reflec-tion arising from the innermost regions of the accretion disc, andComptonization from a warm corona.

There are a number of reasons (e.g. high BH spin and highionization parameters) that could lead to a weak broad iron line,but still a prominent relativistic reflection continuum, particu-larly in the soft X-rays. Indeed, Walton et al. (2013), analysingSuzaku data, found a good fit modelling the NGC 7469 softexcess using a relativistic reflection model. Thus, even if ourprevious analysis failed to find significant signatures from rel-ativistic effects in the hard X-ray band (and, notably, in the ironline profile), we tested for a relativistic origin for the soft excessin this source.

To perform this test, we again added relxill to the model,similar to what previously had been carried out in the NuSTARdata alone, leaving the ionization parameter, coronal emissivity,BH spin, and the normalization free to vary among the observa-tions, while the photon index and high energy cut-off are linkedto those in xillver. We get parameters (ξ = 2.4 ± 0.2, i = 45◦,emissivity = 4.8 ± 0.3, a > 0.996) consistent with those foundby Walton et al. (2013), but our fit is not statistically acceptable(χ2 = 6036 for 2788 d.o.f.). This discrepancy is likely due to themuch higher S/N of our data (especially in the soft X-rays) withrespect to that used by Walton et al. (2013).

We finally tried nthcomp (Zdziarski et al. 1996; Zycki et al.1999), accounting for a Comptonized continuum from a warmcorona, as discussed by Petrucci et al. (2013), Rózanska et al.(2015), and Petrucci et al. (2018). For this model we untieand let free to vary the electron temperature and seed pho-tons temperature among the observations. For each observation,all the parameters among XMM-Newton and NuSTAR are tiedduring the fit, however we need to allow for different values

Table 5. Best-fit parameters for the soft excess, where kTwc is theelectron temperature of the warm corona.

Obs. Γsoft kTwc τwc Normnthcomp[keV] (10−3)

1 2.73+0.02−0.02 0.63+0.04

−0.03 8.8+0.5−0.5 5.77+0.01

−0.01

2 2.66+0.02−0.02 0.71+0.04

−0.03 8.7+0.7−0.6 5.42+0.01

−0.01

3 2.62+0.02−0.02 0.65+0.04

−0.03 9.4+0.8−0.6 4.24+0.01

−0.01

4 2.65+0.02−0.02 0.68+0.03

−0.03 9.0+0.5−0.4 4.10+0.01

−0.01

5 2.58+0.02−0.02 0.68+0.03

−0.05 9.6+0.3−0.7 5.11+0.01

−0.01

6 2.61+0.02−0.02 0.65+0.04

−0.03 9.5+0.6−0.6 4.56+0.01

−0.01

7 2.62+0.02−0.02 0.69+0.03

−0.03 9.2+0.6−0.6 5.15+0.01

−0.01

Notes. The obtained seed photons temperature is ∼3 eV with no signa-ture of any variability among the observations. The best-fit parametersfor the hard X-ray model components are consistent, within the errors,with those obtained in the 4–78 keV band, and are not reported here forthe sake of simplicity.

of photon index between XMM-Newton and NuSTAR in everyobservation. XMM-Newton slopes are harder than the NuSTARderived slopes and this discrepancy, likely due to residual inter-calibration issues, is, on average, of the order of ∆Γ ∼ 0.17 (seee.g. Cappi et al. 2016, and Appendix A for more details). Inthis paper, we report values of the photon index derived fromNuSTAR data.

Following this procedure, we obtained a very good fit to thewhole dataset, with χ2 = 3041 for 2765 d.o.f. (see Fig. 2, bot-tom right-hand side panel). The parameters of the hard X-raycomponents are fully compatible with those obtained from thefit of the data above 4 keV (Sect. 4.2 and Table 2). We reportthe best fit values for the parameters describing the soft excess inTable 5. Most of the observed variability can be attributed to thenthcomp normalization that varies among all the observations.On the other hand, the electron temperature is found consistentwith being constant, while for the photon index marginal varia-tions are observed; see bottom right-hand side panel of Fig. 5.The measured warm corona temperature (kTwc) is found to be,on average, 0.67 ± 0.03 KeV.

The obtained electron temperature can be used to estimatethe optical depth τ for the warm corona. Following Beloborodov(1999), and using his Eq. 13 and the average values for the kTwcand Γ reported in Table 5, we estimate the NGC 7469 warmcorona τwc to be 9.2 ± 0.2 .

6. Discussion

6.1. Two-corona scenario

The broad-band X-ray spectrum of NGC 7469 shows the pres-ence of two main components, the primary power law at highenergies and a strong soft excess which starts dominating below∼4 keV. This is commonly found in Seyfert galaxies (e.g.Piconcelli et al. 2005; Bianchi et al. 2009; Scott et al. 2012).

The high energy X-ray spectrum can be phenomenologicallycharacterized by a cut-off power law with average spectral indexΓ = 1.78 ± 0.02 and high energy cut-off Ecut = 170+60

−40 keV.

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These parameters are consistent with being constant among allthe observations, with only some marginal evidence of variabil-ity of the cut-off energy. The latter value is compatible within theerrors with measures based on Suzaku data (Ecut = 119+65

−31 keV;Patrick et al. 2011) and BeppoSAX (Ecut ∼ 150 keV; De Rosaet al. 2002). On the other hand, the rapid Γ variability reportedby Nandra et al. (2000) could be due to the contamination fromthe soft excess, which could not be properly modelled in RXTEdata. Indeed, it is clear from our monitoring that the soft X-rayenergy part of the spectrum varies more than the high energypart, generating variations of the hardness ratio, which couldmimic a photon index variability if the two spectral componentsare not properly modelled separately (see e.g. Figs. 1 and 2,top left panel). However, the rapid Γ variability reported byNandra et al. (2000) could also be due to a different state ofNGC 7469 at the epoch of the IUE/RXTE campaign. In fact,a comparison between our data and those studied by the authorsreveals that the source was in a more variable state comparedto 2015.

The cut-off power law that reproduces the high energyspectrum of NGC 7469 can be naturally ascribed to Comp-tonization of the accretion disc photons onto a corona of hotelectrons. Adopting a self-consistent Comptonization model, werecovered an average electron temperature of kThc = 45+15

−12 keVand, under the assumption of a spherical geometry, an opticaldepth τhc = 2.6 ± 0.9. These values are within ranges generallyfound in other Seyfert galaxies (e.g. Petrucci et al. 2013, 2018;Tortosa et al. 2018b) and are consistent with being constantamong the observations of our monitoring campaign, which isin agreement with what found with phenomenological models.Interestingly, the coronal parameters measured in NGC 7469lie along the kTe−τ anti-correlation found by Tortosa et al.(2018b) in a sample of Seyfert galaxies observed by NuSTAR.As discussed in their paper, this anti-correlation is suggestive ofvariations in the heating-cooling ratio of the corona, as a resultof different disc-corona geometries and/or intrinsic disc emis-sion. The soft excess cannot be satisfactorily modelled by simplephenomenological models, such as a steep power law or a blackbody, as often found in large samples of objects and/or low S/Nspectra (Bianchi et al. 2009; Matt et al. 2014; Ursini et al. 2015).Moreover, a self-consistent model in terms of blurred relativisticreflection is also statistically unacceptable for reproducing thissoft excess.

An additional Comptonized spectral component provides agood representation of the soft excess in this source. Assum-ing the same seed photons as for the hot corona (i.e. thosearising from the accretion disc), the temperature of the elec-tron cloud responsible for the Comptonization of the spectrumis on average kTwc = 0.67 ± 0.03 keV and the optical depthτwc = 9.2 ± 0.2, again with marginal evidence for variabilityamong the observations of our campaign. These values are wellin agreement with those found in a sample of Seyfert galax-ies, within the framework of the so-called two-corona model(e.g. Petrucci et al. 2013, 2018; Mehdipour et al. 2015; Rózanskaet al. 2015). According to this model, the soft X-ray emissionis produced by Comptonization of the disc photons by a warmoptically thick and extended medium (the warm corona), whichis different from the compact, optically thin, and hot medium(the hot corona) that is responsible for the high energy emis-sion. The ranges of the coronal values found for the warm coronain Seyfert galaxies (including NGC 7469) are consistent withthe warm corona covering a large percentage of a quasi-passiveaccretion disc, whose intrinsic emission is negligible becausemost of the accretion power is released in the warm corona itself

(Petrucci et al. 2018). It is important to note that this two-coronascenario for NGC 7469 is also consistent with the observedoptical-UV emission of this object by HST and Swift UVOT,as quantitatively shown by Mehdipour et al. (2018).

6.2. Reprocessed components

Along with the main continuum components discussed in theprevious section, all the XMM-Newton and NuSTAR spectra ofour campaign on NGC 7469 are characterized by the presence ofa prominent emission line that was readily identified as a neutralFe Kα fluorescent line. We find that this line is unresolved, hasno evidence of any broadening and a flux compatible with beingconstant among all the observations, and with the past XMM-Newton observation in 2000 (Blustin et al. 2003). As expectedif originating in Compton-thick matter, the iron line is associ-ated with a reflection component whose flux is also consistentwith being constant among the observations of our campaign.Consequently, its reflection fraction with respect to the primarycontinuum is slightly variable (R being in the range 0.3−0.6), andhigher values are measured when the primary flux is lower. Self-consistent reflection models agree with a scenario in which boththe iron line and reflection component arise from Compton-thickmatter far from the accretion disc, as commonly found in theX-ray spectra of Seyfert galaxies (e.g. Bianchi et al. 2009; Cappiet al. 2016; Tortosa et al. 2017).

As already noted, we find no evidence for relativistic effectsin the iron line profile of NGC 7469. However, it has to be com-pared with the rich literature for this source. Guainazzi et al.(1994) found the Fe Kα line to be narrow in a 40 ks ASCA spec-trum. In particular, the authors estimated the line-emitting regionto be at several tens of Schwarzschild radii from the centralengine of the galaxy. This result was confirmed by Nandra et al.(1997) with the same dataset. Subsequently, Blustin et al. (2003)working on XMM-Newton data observed the Fe Kα line to be nar-row, and they modelled it with a single narrow Gaussian line. Onthe other hand, the analysis of BeppoSAX data by De Rosa et al.(2002), pointed to a relativistically broadened component of theline, together with an unresolved core. The presence of a rela-tivistic component was confirmed, albeit marginally, by Suzaku(Patrick et al. 2011; Mantovani et al. 2016).

Although we cannot exclude the presence of a broad com-ponent of the iron line in past observations, we may speculatethat the narrow core we observe in XMM-Newton data may becontaminated by other emission lines in spectra with lower spec-tral resolution and/or S/N. Indeed, other emission features areclearly present in the XMM-Newton spectra, as shown in Fig. 3:a strong Fe XXVI Lyα emission line, which was significantlydetected in all the observations of our campaign, and weakeremission lines such as Fe XXV Heα, neutral Fe Kβ and Ni Kα,which were not always significant. While the latter two emis-sion lines are expected to accompany the neutral Fe Kα emission,and therefore share the same origin, the other lines must arisein a much more ionized plasma. Such lines are often observedboth in Seyfert 1s and in Seyfert 2s and are likely producedin a Compton-thin, photoionized material illuminated by thenuclear continuum (e.g. Bianchi & Matt 2002; Bianchi et al.2005; Costantini et al. 2010).

7. Conclusions

In this paper we reported the spectral analysis of seven simul-taneous NuSTAR and XMM-Newton observations of the SeyfertGalaxy NGC 7469 performed from June 2015 to December 2015

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in the context of a multi-wavelength campaign. In the following,we summarize the results of our analysis:

– NGC 7469 displayed a significant flux variability during thisobservational campaign with intra-observation variability ata timescales of few ks. We quantified this variability usingthe normalized excess variance estimator (e.g. Ponti et al.2012) σ2

rms = 0.0021 ± 0.0005; this also allowed us to esti-mate the BH mass to be MBH = 1.1 ± 0.1 × 107 M�, whichis in agreement with the measure based on reverberationmapping (Peterson et al. 2014).

– The high energy spectrum can be phenomenologically char-acterized by a cut-off power law with average spectral indexΓ = 1.78 ± 0.02 and high energy cut-off Ecut = 170+60

−40 keV.These parameters are consistent with being constant amongall the observations, with only some marginal evidenceof variability of the cut-off energy. Using a realisticComptonization model, the derived coronal parametersare kThc = 45+15

−12 keV and τhc = 2.6 ± 0.9 for a sphericalgeometry.

– A strong soft excess is observed in all the observations,extending up to 4 keV. The best description for this com-ponent is through another Comptonized spectrum that isproduced by a warm corona with kTwc = 0.67 ± 0.03 keVand τwc = 9.2 ± 0.2, again with only marginal evidence forvariability among the observations of our campaign. Indeed,most of the observed variability of the soft X-ray data maybe simply ascribed to variations of the normalization of thiscomponent. The overall scenario is consistent with the so-called two-corona model (e.g. Petrucci et al. 2013, 2018;Rózanska et al. 2015), where most of the accretion poweris released in a warm optically thick and extended mediuminstead of the accretion disc.

– A neutral Fe Kα emission line is present in all the observa-tions. The line is found to be narrow, with no indicationsfor relativistic broadening, and consistent with being con-stant. An accompanying Compton reflection component isalso found to be constant among the observations, whichis in agreement with a scenario in which both componentsarise from Compton-thick matter located far away from thecentral BH. Weak neutral Fe Kβ and Ni Kα emission lines,which were only detected in some observations, have thesame origin.

– A Fe XXVI Lyα emission line is significantly detected in allthe observations of this campaign, and is likely to arise in aphotoionized material illuminated by the central continuum,together with a weaker Fe XXV Heα emission line.

Acknowledgements. We thank the referee for helping us improve the quality ofthis paper. This work has made use of data from the NuSTAR mission, a projectled by the California Institute of Technology, managed by the Jet PropulsionLaboratory, and funded by the National Aeronautics and Space Administration.We thank the NuSTAR Operations, Software and Calibration teams for supportwith the execution and analysis of these observations. This research has madeuse of the nustardas jointly developed by the ASI Science Data Center (ASDC,Italy) and the California Institute of Technology (USA). The work is also basedon observations obtained with XMM-Newton, an ESA science mission withinstruments and contributions directly funded by ESA Member States and theUSA (NASA). RM and SB acknowledge financial support from the EuropeanUnion Seventh Framework Programme (FP7/2007-2013) under grant agreementno. 312789. SB acknowledges financial support from the Italian Space Agencyunder grant ASI-INAF I/037/12/0. POP acknowledges financial support fromthe CNES french agency and the CNRS PNHE. GP acknowledges support bythe Bundesministerium fur Wirtschaft und Technologie/Deutsches Zentrumfur Luftund Raumfahrt (BMWI/DLR, FKZ 50 OR 1408) and the Max PlanckSociety. SRON is supported financially by NWO, the Netherlands Organi-zation for Scientific Research.BDM acknowledges support from the PolishNational Science Center grant Polonez 2016/21/P/ST9/04025. The research

at the Technion is supported by the I-CORE programme of the Planning andBudgeting Committee (grant number 1937/12). EB acknowledges funding fromthe European Union’s Horizon 2020 research and innovation programme underthe Marie Sklodowska–Curie grant agreement no. 655324. M.C. acknowledgesfinancial contribution from the agreement ASI-INAF n.2017-14-H.O

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Appendix A: Γ discrepancy between XMM-Newtonand NuSTAR

We reported in our analysis that our simultaneous XMM-Newtonand NuSTAR data yielded photon indexes differing on average of∼0.17. In this appendix, we report our investigations concerninga plausible origin for this issue.

– Different energy band: the energetic range of NuSTAR isdifferent with respect to that of XMM-Newton, thus we per-formed a simple analysis on the same observational band forboth the satellites. For the whole sample of observations,we try to fit the spectra with a simple power law model in

the common energy band 4–10 keV. The measurements forthe photon indexes we obtained were still discrepant of thesame amount.

– Pile-up: our XMM-Newton spectra could, at least marginally,suffer from pile-up. According to the XMM-Newton handguide3, pile-up can affect pn observations performed inSmall Window mode, when a total count rate of ∼25 c/sis exceeded. Therefore, at least the first and second XMM-Newton observations could suffer from pile-up problems.

– We thus tried to extract the spectra from source annularregions, using different values for the annulus inner radius(from 50 up to 200 pixels). However, our tests show thatany choice of the inner annular radius improves the ∆Γdiscrepancy between the XMM-Newton and NuSTAR spectra.

– Intrinsic variability: XMM-Newton observations are longerwith respect to those performed by NuSTAR, thus the differ-ence in the two Γ could be due to the not truly simultaneityof the observations. To verify this hypothesis we extractedXMM-Newton spectra exactly in the same temporal rangeof those obtained using NuSTAR4. As in the previous cases,analysing these truly simultaneous spectra does not affect the∆Γ discrepancy.

We therefore conclude that the most likely origin for the spectralindex discrepancy has to be found in residual inter-calibrationissues between XMM-Newton and NuSTAR5, whose significativ-ity may vary from observation to observation, and whose impacton the analysis is larger for high S/N data. We estimate that theresults in this paper are not qualitatively affected by this issue,but a systematic uncertainty in the reported best-fit parametersshould be taken into account.

3 https://xmm-tools.cosmos.esa.int/external/xmm_user_support/documentation/uhb/epicmode.html4 For the fourth observation, the two satellites do not observeNGC 7469 simultaneously so that we cannot perform this test on it.5 A new SAS version was released when we were finalizing this paper(SAS version 16.1). We performed several tests to check if this couldaffect our results, but found that the reported ∆Γ between XMM-Newtonand NuSTAR is not significantly improved.

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